Analog Devices LTC3302A Manuel utilisateur

User Guide | DC3251A

UG-2105

Evaluating the LTC3302A, 3.3 V to 1.2 V at 2 A, 2 MHz Synchronous Step-Down Regulator in a 2 mm × 2 mm SWF FCQFN

PLEASE SEE THE LAST PAGE FOR AN IMPORTANT

WARNING AND LEGAL TERMS AND CONDITIONS. Rev. 0 | 1 of 13

FEATURES

►LTC3302A evaluation board

►Transient circuit included for load-transient evaluation

►EMI filter included to reduce noise in EMI emission tests

EVALUATION KIT CONTENTS

►DC3251A evaluation board

DOCUMENTS NEEDED

►LTC3302A data sheet

EQUIPMENT NEEDED

►One 5 V, 2 A, DC power supply

►One 2 A, Electronic load

►Three digital voltmeters

►Two digital ammeters

GENERAL DESCRIPTION

Demonstration Circuit DC3251A features the LTC3302A, 5 V, 2 A

synchronous step-down silent switcher operating as a 2 MHz, 3.3

V to 1.2 V, 2 A buck regulator. The LTC3302A supports adjustable

output voltages from 0.5 V to VIN. The LTC3302A is a compact,

high efficiency, and high-speed synchronous monolithic step-down

switching regulator. A minimum on-time of 27 ns enables high VIN to

low VOUT conversion.

The DC3251A operating mode can be selected as Burst Mode

operation, skip (PS) or forced continuous (FC) mode. Setting JP1

to the SKIP position allows the LTC3302A to sync to a clock

frequency from 1.6 MHz to 2.4 MHz. The LTC3302A operates in

forced continuous mode when syncing to an external clock.

The DC3251A also has an EMI filter to reduce conducted EMI. This

EMI filter can be included by applying the input voltage at the VIN

EMI terminal. The EMI performance of the board is shown in the

EMI Test Results section. The red lines in the EMI performance

graphs show the CISPR25 Class 5 peak limits for the conducted

and radiated emission tests.

The LTC3302A data sheet gives a complete description of the

device, operation, and application information. Full specifications on

the LTC3302A are available in the LTC3302A data sheet available

from Analog Devices, Inc., and must be consulted with this user

guide when using the DC3251A evaluation board. The LTC3302A is

assembled in a 2 mm × 2 mm FCQFN package with side wettable

flanks (SWF) for visual solder inspection. The layout recommenda-

tions for low EMI operation and maximum thermal performance are

available in the data sheet section Low EMI PCB Layout.

Figure 7 shows the efficiency and the power loss of the circuit with

a 3.3 V input in Burst Mode operation.

User Guide DC3251A

TABLE OF CONTENTS

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Features................................................................ 1

Evaluation Kit Contents......................................... 1

Documents Needed...............................................1

Equipment Needed................................................1

General Description...............................................1

DC3251A Evaluation Board Photograph............... 3

Performance Summary..........................................4

Quick Start Procedure........................................... 5

Typical Performance Characteristics..................... 8

EMI Test Results....................................................9

Evaluation Board Hardware.................................10

Introduction to the DC3251A............................10

Accurately Measuring Output Ripple of the

LTC3302A...................................................... 10

Evaluation Board Schematic................................11

Ordering Information............................................12

Bill of Materials.................................................12

REVISION HISTORY

3/2023—Revision 0: Initial Version

User Guide DC3251A

DC3251A EVALUATION BOARD PHOTOGRAPH

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Figure 1. DC3251A Evaluation Board Photograph

User Guide DC3251A

PERFORMANCE SUMMARY

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Specifications are at TA = 25°C, unless otherwise noted.

Table 1. Performance Summary

Parameter Symbol Test Conditions/Comments Min Typ Max Unit

VOLTAGE RANGE

Input VIN 2.25 5.5 V

Output1VOUT 1.183 1.2 1.217 V

OUTPUT CURRENT IOUT 2 A

SWITCHING FREQUENCY fSW VIN > VOUT 1.6 2.0 2.4 MHz

TOP SWITCH MINIMUM ON-TIME tON 27 ns

TOP SWITCH DUTY CYCLE 100 %

1With 1% resistors. Accuracy improves to within 1% using 0.1% FB resistors or a fixed voltage version of the LTC3302A.

User Guide DC3251A

QUICK START PROCEDURE

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Demonstration circuit DC3251A is easy to set up and use to evalu-

ate the performance of the LTC3302A. For the proper measurement

equipment setup, see Figure 3 and follow the procedure below:

NOTE: For accurate VIN, VOUT, and efficiency measurements,

measure VIN at the VIN SNSE and GND SNSE turrets, and VOUT

at the VOUT SNSE and GND SNSE turrets as shown as VM1 and

VM2 in Figure 3. When measuring the input or output voltage ripple,

care must be taken to avoid a long ground lead on the oscilloscope

probe. Measure the output voltage ripple by touching the probe

tip directly across the output turrets or to TP1 as shown in Figure

4. TP1 is designed for a 50 Ω coax cable to reduce any high

frequency noise that might couple into the oscilloscope probes.

1. Set the JP1 Jumper to the SKIP position and JP2 to the HI

position.

2. With power off, connect the input power supply to VIN and GND.

If the input EMI filter is required, connect the input power supply

to VIN EMI.

3. Slowly increase PS1 to 1.0 V. If AM1 reads less than 20 mA,

increase PS1 to 3.3 V. Verify that VM1 reads 3.3 V and VM2

reads 1.2 V.

4. Connect an oscilloscope voltage probe as shown in Figure 4

in parallel with VM2. Set Channel to AC-coupled, voltage scale

to 20 mV and time base to 10 µs. Observe the VOUT ripple

voltage.

5. Verify that PGOOD turret is above 1 V.

6. Increasing the load by 1 A intervals up to 2 A and record VM1,

VM2, AM1, and AM2 for each interval.

7. Repeat step 6 for PS1 set to 2.5 V and again for PS1 set to 5.0

8. Set the load to a constant 1 A. Remove the oscilloscope voltage

probe from VOUT. Place a ground clip on PGND terminal and

set the voltage scale to 1 V and the time scale to 500 ns/

Division. Trigger on the rising edge of the voltage probe. Using

a tip on the voltage probe, contact the SW node on the pad

of L1. Observe the duty cycle and the period of the switching

waveform (~500 ns).

9. Set the load current to 0.1 A and repeat step 8. Observe that

the switching waveform is now operating in pulse skip mode.

10. Move the jumper on JP2 to LO. Verify that VOUT reads 0 V

and verify that PGOOD is low. Return jumper on JP2 to HI and

verify that VM2 is 1.2 V and verify that PGOOD is above 1 V.

11. If forced continuous or Burst Mode is required, set PS1 to 0

V. Move JP1 to FC or BURST. Repeat steps 3 through 9. In

step 9, observe that the switching waveform is now operating in

forced continuous or Burst Mode.

12. To test the transient response with a base load, add the re-

quired resistor to produce a minimum load between VOUT and

RSNS turrets (RL shown on Figure 3). Note that the total load

resistance is RL plus R8 (100 mΩ).

13. Adjust a signal generator with a 10 ms period, 10% duty cycle,

and an amplitude from 1 V to 2 V to start.

14. Measure the RSNS voltage to observe the current, VRSNS/100

mΩ. Adjust the amplitude of the pulse to provide the required

transient. Adjust the rising and falling edge of the pulse to

provide the required ramp rate. For more details, see Figure 9

and the optional transient response circuit shown in Figure 6.

IOUT =VRSNS/100mΩ

(1)

15. When done, turn off PS1 and Load. Remove all connections to

the demo board.

User Guide DC3251A

QUICK START PROCEDURE

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Figure 2. DC3251A Simplified Schematic

Figure 3. Test Setup for the DC3251A Demo Board

Figure 4. Technique for Measuring Output Ripple and Step Response with a Scope Probe

User Guide DC3251A

QUICK START PROCEDURE

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Figure 5. Technique for Measuring Output Ripple and Step Response with a Low Inductance Connector (Not Supplied)

Figure 6. Optional Transient Response Circuit

User Guide DC3251A

TYPICAL PERFORMANCE CHARACTERISTICS

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Figure 7. Efficiency and Power Loss in Burst Mode Operation

Figure 8. Buck Load Regulation

Figure 9. Load Transient Response Forced Continuous Mode

User Guide DC3251A

EMI TEST RESULTS

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Figure 10. Conducted EMI Performance (CISPR25 Conducted Emission Test

with Class 5 Peak Limits)

Figure 11. Radiated EMI Performance (CISPR25 Radiated Emission Test with

Class 5 Peak Limits, Vertical)

Figure 12. Radiated EMI Performance (CISPR25 Radiated Emission Test with

Class 5 Peak Limits, Horizontal)

User Guide DC3251A

EVALUATION BOARD HARDWARE

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INTRODUCTION TO THE DC3251A

The DC3251A demonstration circuit features the LTC3302A, a

low voltage synchronous step-down regulator. The LTC3302A is

a monolithic, constant frequency, current mode step-down DC-DC

converter. A 2 MHz oscillator turns on the internal top power switch

at the beginning of each clock cycle. Current in the inductor then

increases until the top switch comparator trips and turns off the top

power switch. If the EN pin is low, the LTC3302A is in shutdown

and in a low quiescent current state. When the EN pin is above its

threshold, the switching regulator is enabled.

The MODE/SYNC pin sets the switching mode to pulse skip, forced

continuous, or Burst Mode. If an external 1.6 MHz to 2.4 MHz clock

is connected to the MODE/SYNC turret while the JP1 is set to

the SKIP position, the LTC3302A switching frequency sync to the

external clock. The LTC3302A operates in forced continuous mode

while syncing. For more detailed information, refer to the LTC3302A

data sheet.

ACCURATELY MEASURING OUTPUT RIPPLE

OF THE LTC3302A

With the fast edge rates of the circuit, high frequency noise can

be observed when measuring the output voltage with 1 MΩ termi-

nated oscilloscope probes. To better view the output ripple with

oscilloscopes of 400 MHz bandwidth and above a 50 Ω coax cable

connected as close to the output caps as possible should be used

with the oscilloscope channel terminated to 50 Ω at the scope.

This helps to reduce the noise coupling onto and displaying on the

scope. The demo board is set up to solder an U.FL, RECEPT, ST

SMD, 0 Hz to 6 GHz, 50 Ω connector (TP1) near the output cap

C6. These pads can also be used to solder a coax cable or other

oscilloscope probe connector if required.

The high frequency spikes are partially attributed to the inter-wind-

ing capacitance of the inductor and the voltage step is partially

attributed to the inductance in the output capacitors. This can

be reduced by choosing low ESL capacitors or adding small low

ESL capacitors in parallel to the output capacitors as close to the

inductor as possible. Figure 13 and Figure 14 show the output

ripple using a 500 MHz scope, 50 Ω probe with an added low ESL

X2Y capacitor added, C9, close to the inductor and GND return to

the input capacitors to reduce the inductance of the return path and

better filter the high frequency spikes.

Figure 13. VOUT Ripple Without C9 X2Y Capacitor

Figure 14. VOUT Ripple with C9 X2Y Capacitor

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